Hi and welcome to windows central.
have you recently upgraded your phone to windows 10? it is extremely important to hard reset your device once after upgrading to windows 10.
Have you thoroughly checked what the sound is like or is there a notification that pops up, many a times what has happened with me is when i plug in my earplugs my lumia gives me a warning notification that " listening to music for long hours is bad " this could be happening to yours as well. Please tell us more.
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